A low noise amplifier is often used to initially amplify low level signals, such as radio frequency signals used in communication systems. Desirable properties in a low noise amplifier include a low noise figure and a high gain. FIG. 1 is a schematic diagram of one embodiment of prior art low noise amplifier 100. Low noise amplifier 100 includes transistors 103 and 106, inductors 109 and 112, and impedance matching network 115. Signal source 118 is coupled to impedance matching network 115. The output signal from impedance matching network 115 is coupled to transistor 103. Transistor 103 is connected in series with transistor 106 and inductors 109 and 112. Inductor 109 functions as a direct current bias conduit and radio frequency load while assisting in tuning out the input capacitance of the following stage. Inductor 112 is capable of tuning the input impedance of transistor 103 to match the signal source impedance while degenerating the source to improve the linearity of the amplifier.
Tuning affects the noise figure, gain, and stability of a low noise amplifier, so tuning is an important feature of the input stage of a low noise amplifier. Inductor 112 alone is not sufficient to provide precise tuning for low noise amplifier 100, since the value of the parasitics that inductor 112 is attempting to tune are typically not known and are difficult to characterize experimentally. A value for inductor 112 is chosen to provide "coarse" tuning of the input impedance. Impedance matching network 115 is added to provide "fine" tuning. Unfortunately, impedance matching network 115 requires the use of expensive off-chip components. Since the inductor values capable of being realized in complementary metal-oxide semiconductor (CMOS) process are poorly controlled and restricted to a very narrow range, the use of off-chip components is common in the fabrication of low noise amplifiers using CMOS processes. Therefore, inductor 112 cannot be used alone to precisely tune low noise amplifier 100 to the source impedance.
For these an other reasons there is a need for the present invention.